1. Field of the Invention
The invention relates to a dewatering method for correcting water contents of green veneers for plywood. In the dewatering method, a pressure is vertically applied to a large number of vertically laminated green veneers for plywood so that the green veneers are compressed and dewatered for reduction of water contents of the green veneers. The invention also relates to an apparatus for dewatering the green veneers.
2. Description of the Related Art
Describing a plywood production process often employed, a log being rotated is peeled into thin band plates by the use of a lathe (plane), green veneers are obtained by cutting the peeled band plates in predefined dimensions and then dried, and a plurality of the green veneers is bonded to produce the plywood. In the process, the green veneers are compressed and dewatered by applying a pressure to the green veneers before the drying step because it is time-consuming to immediately dry the obtained green veneers.
The water contents of the respective green veneers thus dewatered are desirably as equal as possible not only among the laminated green veneers but also in different parts of each green veneer. A large difference between the water contents leads to a lengthy drying step and makes it difficult for the green veneers to be equally dried, in which case the water contents are still different from one green veneer to another and in different parts of one green veneer after the drying step. When the green veneers thus having different water contents are laminated to produce the plywood, warp and/or distortion is likely to occur.
Comparing sapwood and core pieces respectively obtained from outer layers and a core part of the log, the water contents of each green veneer before compressing step are largely different. For example, the water content of the sapwood is twice to three times as large as a relatively low water content of the wood core. When the sapwood and core pieces, each one of which is inadequate and unusable as a veneer, are joined and used as a patched veneer, the veneer may have water contents two-fold to three-fold different from one another in different parts of the patched veneer. In some of such green veneers, there may be differences by several-ten percentages or more among the water contents in different parts of one green veneer. The Applicant was granted two patents in Japan last year, U.S. Pat. No. 4,783,862 and U.S. Pat. No. 4,783,863 (both registered on Jul. 15, 2011). This invention is an independent invention distinctly different from these two patented inventions.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2002-166403
[Patent Document 2] Japanese Laid-Open Patent Publication No. 2010-197004
The Patent Document 1 discloses a roller dewatering apparatus. In the apparatus, green veneers are each transported through between squeeze rollers to be thereby pressed and dewatered. This apparatus which dewaters the green veneers one by one needs an extensive time for its whole operation. Further, the apparatus, wherein all of the green veneers are uniformly pressed by the rollers, is unable to correct a variability of water contents among the different green veneers.
The Patent Document 2 discloses a dewatering apparatus configured for laminate collective compression. In the apparatus, laminated green veneers are pressed collectively from an upper direction to be dewatered. The apparatus thus dewatering all of the green veneers at once achieves a better dewatering efficiency. However, the apparatus is similarly unable to correct a variability of water contents, whether they are different water contents among the green veneers and/or different water contents in different parts of one green veneer.